Support for multiple sessions

• jrc now supports multiple connections to a single server and thus can be used to create server apps that are intended
  to be used by multiple clients simultaneously. This change requires some additional arguments in some of the functions as well
  as several new ones. However, backwards compatibility is maintained.

• jrc now depends on R6 and each app is represented with a single object. One can manage the app with methods of this object,
  which allows to run several apps inside one R session. For more information, check man pages of classes App and Session.